Super strength steel alloy composition and product and process of preparing it



y 24, 1956 I J. P. TARWATER 3,252,840

SUPER STRENGTH STEEL ALLOY COMPOSITION AND PRODUCT AND PROCESS OFPREPARING IT Original Filed Sept. 21, 1961 5 Sheets-Sheet l Tia. 1..

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ll 5 (WITH Warm/1v f57'E/4/A/ yam/a 300 1 f K w/r/mw' PEESrfiA/A/AA/D ,4ELEV/7 TEDJEMPEEATUFE Q ave/2w AG/A/G m f I/ 1/ 4 1 g 0 E u loo STR/U/VINVENTOR. Jh/wis P/QRWATER May 24, 1966 J. P. TARWATER SUPER STRENGTHSTEEL ALLOY COMPOSITION AND PRODUCT AND PROCESS OF PREPARING IT OriginalFiled Sept. 21, 1961 3 Sheets-Sheet 2 W 53k E QQMRQE max m LI Q Q a x 75 pm (1252 10 2/, z '0) #1 a/vawy 475M INVENTOR. M44455 74mm 75/? BYMOM/41% ,4 U'OF/ E y 24, 1966 J. P. TARWATER SUPER STRENGTH STEEL ALLOYCOMPOSITION AND PRODUCT AND PROCESS OF PREPARING IT Original Filed Sept.21, 1961 5 Sheets-Sheet 5 8 (MIC/{55) 0 Z M m fl n f m w Y B UnitedStates Patent This application is a division of copending applicationSerial No. 140,072, filed September 21, 1961, now Patent No. 3,198,630.

This invention relates to super strength steel alloy composition andproduct and process of preparing it. More particularly the presentinvention relates to a steel alloy composition capable of being treatedto produce a super strength body as one having a tensile strengthsubstantially above 300,000 p.s.i., while having a substantiallydefinite yield point which is substantially the same as the ultimatetensile strength and further, while having a high degree of ductility.

The invention further relates to the provision of an alloy steelcomposition of the super strength type, which will have a very highas-tempered strength attained over a substantial range of temperingtemperatures, as from about 400 F. to about 900 R, such compositionpreferably including cobalt and/or aluminum.

The present invention further provides a super strength steel alloycomposition which will have high fracture toughness as measured by theresistance of a notched sample of sheet material to crack propagationunder stress.

Super strength steels have now become a recognized group of steelalloys, so that various compositions have been disclosed by differentmanufacturers of such steels along with their claimed tensile strengths,which in practically all instances are substantially less than 300,000p.s.i. In most instances these steels are in the medium carbon range andinclude such other .alloying elements as manganese, silicon, chromium,molybdenum, vanadium and nickel. In practically all instances, however,the nickel content has been quite low, rarely above 2%; and some otherof the elements aforesaid have been present in amounts which aresubstantially above those contemplated as tolerable in accordance withthe present invention, for example, chromium has usually beensubstantially higher than the upper limit given in the present inventionof about 0.5%. The present invention by contrast with. these may betermed a medium carbon nickel steel wherein the nickel present is fromabout 3 to 7% and for certain purposes, further limitations ashereinafter set forth are imposed upon the composition.

The prior art, for example, has also suggested a number of suchcompositions of what are referred to generally in the art as Ladish-typesteels, certain of these being set out in US. Patents Nos. 2,919,188 and2,921,849, both owned "by Ladish Company, Cudahy, Wisconsin. Thesesteels have a quite low nickel content, it being stated, for example, inthe first of these patents:

The nickel content is always less than the chromium content and isalways less than the molybdenum content.

As hereinafter stated in greater detail, the compositions of the presentinvention attain results of tensile strength, yield strength andductility which are substantially superior to the Ladish-type steels andto any other known group or type of steels available in accordance withthe prior art teachings, both from the point of view of the as-temperecharacteristics of yield strength and tensile strengths over asubstantial range of tempering temperatures and also from the point ofview of ductility accompanying the high yield strength and high tensilestrength characteristics. Another group of these steels all within thegeneral scope hereinabove outlined has greatly superior fracturetoughness as contrasted with known super strength steels, which is .acharacteristic that is becoming more and more important in some of theuses for which such steels are required today. One such use ofprestrained and strain-aged materials is in the making of high velocityrotors such as are required for high speed precision pumps where closetolerances require. a very high 0.02% offset yield strength. Other usesinclude but are not limited to tensile members of rigging, frames,structures, particularly for aircraft and missiles where highstrength-to-weight. ratios are required.

The prior art has also suggested a process of treating steel which is insome respects essentially similar to the process herein disclosed;except that this process has heretofore been used only on steels havinglow nickel con-tent such, for example, as AISI Type 4340 steel (1.74%nickel) and where the chromium content was substantially higher thanthat as desired or contemplated in accordance with the present invention(i.e. 0.82% chromium). This AISI-Type 4340 steel and the process oftreating it has been described in considerable detail in theTransactions of the American Society of Metals quarterly edition forMarch 1961, pages 72-83. The fact that the resulting steel, while havingsuper strength in tension, was fatally deficient in its ductilitycharacteristics is set out on page 82 of this article wherein it isstated the strength is accompanied by an almost complete loss of stableelongation.

In contrast with this, the steels of the present invention mayadvantageously be treated in accordance with this process, i.e. treatingby austenitizing at a sufliciently high temperature, then quenching,then tempering at a relatively low temperature, followed by a plasticprestrain beyond the elastic limit of the metal, and a subsequenttreatment which is herein referred to as strain-aging in which the steelpreviously subjected to strain as aforesaid, is held at a desiredtemperature for a period of time such that the desired characteristicsare induced and substantially permanently maintained therein.

When a steel alloy composition within the limits set out in the presentinvention is treated by the process aforesaid, the resulting steel bodywill have super strength in the direction in which it was prestrained,accompanied by a yield point which is reasonably definite and with a0.2% offset yield point as hereinafter'defined, which is approximatelyequal to the tensile strength. Such a steel product is per se a part ofthe present invention. This product, however, cannot be distinguishedfrom other steels which do not share the same physical characteristicsor desirable properties by any of the conventional tests. As such,therefore, this product can be defined, as far as O can presently beknown, only by the process of making it, coupled with the compositionthereof, both of which are necessary in order that the product shallhave all the novel and desirable physical characteristics of the presentproduct.

The invention will be better appreciated from a detailed descriptionthereof which follows, and wherein reference is made to the accompanyingdrawings, in which:

FIG. 1 is a comparison of a steel which has been ternpered, but notprestrained or strain-aged as against a steel which is treatedcompletely in accordance with the present invention to include elevatedtemperature strain-aging, the figure being a chart of tensile stressagainst strain and also showing the 0.02% offset line and the 0.2%offset line;

FIG. 2 is a plot of yield strength (0.2% offset) in units of 1000 xp.s.i. against tempering temperature in degrees Fahrenheit;

FIG. 3 is a chart similar to FIG. 2 and for the same set of test samplesof tensile strength against tempering temperature; and

FIG. 4 is a view of a test piece as used for determining the fracturetoughness of a sample of steel by determining the strength level atwhich a crack propagates rapidly in a sharply notched sample.

From a broad point of view, a group of steels within the general limitsas set out will accommodate themselves to three different but alliedobjects and purposes. All these purposes require a super strength steel,ie, one with a very high as-tempered or as-heat-treated tensilestrength. Many high strength steels do not have a true yield point, sothat it has become a custom to get a so-called yield point by firstplotting stress applied to a test piece, for instance, stress intension, against the actual elongation or strain on this test piece.During the period of application of force (stress) below and up to theelastic limit of the material, such a plot is a relatively straight lineinclined upwardly and to the right. Beyond a point substantiallycorresponding to the elastic limit of the metal the plot curves off tothe right with no perceptible sharp break as a curve which is initiallysubstantially tangent to the previous straight line. For this reason ithas become common to draw an arbitrary straight line parallel to thestraight portion of the stress-strain plot that lies below the elasticlimit, and which is offset therefrom by 0.2% in strain or elongation andto take the point of intersection of this arbitrary line with theprincipal curve itself as the yield point or rather as the 0.2% offsetyield strength. The present invention, however, provides steels, whichhave after treatment substantially a true and quite sharp yield point asshown by the comparison of curves A and B on FIG. 1 of the drawings.

The broad composition of steels within the present invention will firstbe discussed. This composition is as follows:

About 0.35-0.55% carbon About -0.5% molybdenum About 37% nickel About0-0.2% vanadium About 0.2-0.5% chromium About 05% cobalt About 02%manganese About 0-1% aluminum About 02% silicon and not over about 0.01%each of sulfur and phosphorus and the remainder being iron withincidental impurities.

Of the ingredients hereinabove listed, the carbon is selected in aso-called medium range of 0.350.60%; as steels having a lower carboncontent than that in the range selected present no real advantage overthe prior art; while steels having a higher carbon content than therange selected are embrittled, so that the desired characteristics ofductility are not present. The preferred range of carbon from the pointof view of providing a steel having a maximum ability to produce adesired prestrained and strain-aged body (to give super strength plusductility) is somewhat narrower in that for this purpose the carboncontent is preferably about OAS-0.50%. It is further 4 noted that thispreferred range carbon content is somewhat higher than that of theAISI-Type 4340 steel (which has 0.40% carbon); and yet the resultantsteel alloy has a much higher ductility as treated in accordance withthis invention.

The next most important alloying ingredient in the steels of the presentinvention is nickel. From a broad point of view this nickel content maybe from about 3 to about 7%. From a more limited point of view, it ispreferred that nickel shall be from about 3 /2 to about 5%, excellentresults having been obtained at both these limits and no reason beingknown why the range therebetween should not give equally excellentresults for many though perhaps not all purposes. The lower limit ofnickel content, however, is quite critical in that when the amount ofnickel present is substantially below 3%, such as in the AISI-Type 4340steel wherein there is a nickel content of only 1.74%, there is very lowductility for the prestrained and strain-aged bodies. The higher limitfor nickel is not as critical, but substantially higher ranges of nickelgive essentially different type alloys, which do not follow generallythe rules nor have the characteristics applicable to the present groupof alloy steels. It is also to be remembered that nickel is much moreexpensive than are some of the other ingredients, particularly the ironingredient which is of course present to a very major extent and,therefore, as the precentage of nickel is greatly increased, the cost ofthe final alloy steel is correspondingly increased.

Manganese is similar in some of the characteristics provided thereby tosilicon, in that both provide some degree of hardenability for the steelalloy. Generally, a residual of manganese is maintained to combine withsulfur so as to prevent hot workability difficulties. However, with ajudicious selection of raw materials, the manganese additions may bereduced or wholly omitted, so that the lower limit may be said to bezero. The maximum of about 2% is chosen, as there is no apparentimprovement in the characteristics of the products with greater amountsof manganese. Thus the upper limit is not a critical limit, but is onedictated to the maximum extent at least by economic factors, rather thanby factors having to do with the technical properties of the product.

Silicon is generally found in many steels to some extent and hasgenerally the function of retarding the tempering reaction attemperating temperatures of 600 or less. Generally, silicon is added tocombine with oxygen in the melt, however, with special meltingtechniques, the silicon may be wholly omitted, so that the lower limitmay be said to be zero. The maximum value of silicon of about 2% ischosen for the reason that as the amount of silicon is increased, thefinal product tends to become more and more brittle. Values greater thanabout 2% thus impart undesired brittleness to the product.

Chromium tends to prevent graphitizing during the heat treating or inservice of the steel alloy bodies and is preferably present in theamounts from about 02-05% in accordance with this invention. Thepreferred concentration range is from about 0.25% to about 0.35%.

It is desired that sulfur and phosphorus be minimized, as it is wellknown in all ferrous metallurgy that sulfur tends to render the partsmade therefrom brittle when hot, While phosphorus make them brittle whencold. The values for sulfur and phosphorus, therefore, are given asmaximum tolerable values throughout this specification; as it isunderstood that the lowest possible values are desirable, but that it isnot practically possible to eliminate these elements altogether undercommercial operating conditions.

Another element which is optionally usable in the composition is cobalt,the outside limits of such use in accordance with the present inventionbeing about 0-5%. Thus it is specifically contemplated that compositionshaving no cobalt at all are to be considered as included .may becollectively termed carbide formers.

in the present invention; While compositions over about are to beconsidered as excluded. The upper limit in this case is not particularlycritical. The function of cobalt, at least in the presence of somesilicon, is to improve the temper resistance and provide desirablephysical characteristics in the material particularly on as as-temperedbasis; in other words, without the steps hereinafter discussed ofprestraining and strain-aging. On the other hand, as hereinafter setout, cobalt-containing alloys have been proven not only useful andoperative, but highly desirable when prestrained and strain-edged.

Aluminum is another optionally usable element, and is desirable for useparticularly in steels containing cobalt as hereinafter set out. As faras is known this element acts as a deoxidizer for the steel compositionsin the relatively small concentrations contemplated in accordance withthe present invention, i.e. from 01%. In this instance also the 0 ismeaningful in that it is specifically contemplated that many steel alloycompositions in accordance with this invention may not contain anyaluminum whatsoever.

Molybdenum and vanadium are also optionally present elements in thatthey may be absent altogether, which is the reason that the lower limitin each instance as to these elements is given as 0. The maximum may betaken as about 0.5% for molybdenum and about 0.2% for vanadium.

It has been found generally that the elements chromium, molybdenum,vanadium, tungsten and columbium They, or some of them, are used inpractically all high strength steels to some extent. It is noted,however, that substantial quantities of members of this group of metalstend to render the steel hard and strong, but with very littleductility. In general they are used in steels, which are to be temperedat above 600 F., and therefore, above the range where super strengthsteels usually exhibit their maximum tensile strength. In the case ofmost of the steels in accordance with the present invention theseelements are kept at or near a minimum, consistent with the desire forstrength, and in order that the resulting alloy steel shall havesubstantial ductility.

The steels of the present invention are characterized more predominantlyby the presence of the noncarbide formers such as nickel, silicon,cobalt and aluminum. It has been found that manganese has some of thecharacteristics of the carbide formers and some of the noncarbidef-ormers, so that it cannot be classed exclusively with either group.

Other elements may be present in relatively small or trace amounts, suchas calcium, copper, titanium, zirconium, columbium, tantalum and boron.However, the amount of any of these metals, if present in the alloy, orthe total of all of them, is so small as not substantially to affect theproperties of the alloy as a whole. As such, these alloys can all beclassed as incidental impurities in the iron if and to the extent thatthey are present at all. It is not a part of the present inventionintentionally to introduce any of these elements as such.

It is contemplated that most if not all metal parts according to thepresent invention will be heat treated at least at first by a more orless conventional heat treating procedure which will include first anaustenitizing step in which the steel alloy is first heated to and heldat a temperature preferably in excess of the range of about 1400-1450 F.for a period of time sufficient to bring the metal to a relativelyuniform and stable condition at this temperature. The metal is thenquenched in oil or in a fused salt bath as hereinafter particularlynoted, this quenching being entirely conventional and hence not beingdescribed in any greater detail. Thereafter the quenched body is usuallytempered by bringing it to and holding it at a selected temperingtemperature which is usually in the range of about 350 to about 600,although in some instances tempering temperatures as high as 800 or moremay be used. It is found, however, that for maximum strength, the lowertempering temperatures are usually desirable, with a maximum usually ofnot over 600 F. and with apreferred tempering temperature for manyalloys according to the present invention of about 400, all temperatureshere given being in degrees Farenheit.

Theusual experience with previously known steels, such as the AlSI-Type4150, is that as the tempering temperature is raised, the yield strengthand the ultimate tensile strength is progressively reduced. Data as to415 O-type steel is shown by the dotted lines G and G in FIGS. 2 and 3respectively. The steels according to the present invention, however,have increasing yield strengths (0.2% otfset points) with increasingtempering temperatures from 400 to 600 and even above 600, the yieldstrengths of preferred compositions are above those for conventionalsteels such as the 4150 type. This is illusstrated best in FIG. 2 of thedrawings showing in graphic form results of testing preferredcompositions according to the present invention with respect to type4150 steel.

The test results are represented by lines C and D on FIG. 2 of thedrawings, the composition of the material tested to produce line Ccontaining both cobalt and aluminum and the composition of the materialtested to produce line D containing cobalt, but no aluminum, both ashereinafter set out in detail. There is also shown on this same figure aline B representing similar data for a composition similar to that usedfor the test forming the basis for lines C and D, but in this casecontaining neither cobalt nor aluminum. These are further to be comparedwith a composition forming the basis for curve P, which not onlycontained no cobalt nor aluminum, but did contain both molybdenum andvanadium, thus having an excess of carbide formers in accordance withthe preferred compositions of this invention. The composition formed onthe basis of line F is relatively undesirable from the point of view ofits as-tempered characteristics, which are those shown in FIGS. 2 and 3.Thus this composition, while being within the invention in that it isadvantageously usable with the specific process of the present inventionincluding prestraining and strain-aging, does not have the desirablecharacteristics necessary for another phase of the invention having todo with a relatively high as-tempered strength and yield strength over asubstantial range of tempering temperatures. All these may further becompared with line G on FIG. 2, which is that for a previously knowntype of steel, namely, No. 4150 steel. For purposes of record, inaccordance with a standard reference book on steel, the composition ofNo. 4150 steel is as follows:

Carbon OAS-0.53%. Manganese 0.75l.00%. Sulfur and phosphorus 0.40%maximum each. Silicon a- 0.20-0.35 Nickel Chromium O.801.l0%. Molybdenum0.15-0.25%.

with the remainder being iron with incidental impurities. In FIG. 3 thelines C, D, E, F and G are drawn with data from stress versus straintests on the same group of steels respectively as the correspondinglynumbered lines on FIG. 2 without the prime marks.

From the data illustrated in FIGS. 2 and 3 it is obvious that while thetensile strengths of the several steels tempered at 400 F. is higherthan those tempered at higher temperatures, the steels of the presentinvention exhibit an unexpected improvement or increase in the 0.2%offset yield strength when tempered up to 600 and higher with respect toprior art steels such as that tested to give the dotted lines G and G.

Continuing now as to the process of the present invention, applicable toa relatively large group of steels all within the present invention, andfollowing the usual tempering step, there are provided prestraining andstrain aging steps. These will now be discussed.

When it is known that a particular part such as a steel piece is to haveto withstand a particular type of externally applied force such astension, compression, twisting or torque in a right hand direction or ina left hand direction, but not more than one of these four kinds offorce (considering right and left hand torque as two kinds), then it ispossible by the process of the present invention to attain superstrength characteristics in the desired one of these four directions.Inasmuch as tension is usually considered as a prime method of testingand many steel parts must be made to withstand tensile forces asdistinguished from either compression, right hand torsion or left handtorsion, then the part in question is prestrained in tension in the samemanner that it is desired to be strong in service. Thus a part which isto withstand tension is prestrained in tension, and the part towithstand compression is prestrained compression, etc. This is necessaryin order to avoid the so-called Bauschinger effect. A very generalsummary of this effect is that while a part which is to withstandtension, for example, may be prestrained in tension and then strainaged; if this part is to withstand compression, prestraining and strainaging in tension is not of significant assistance. Similarly, if a partis to withstand right hand torsion, prestraining and strain aging inleft hand torsion is of no assistance, and in fact, may even render thepart so treated weaker than a wholly untreated part. With this idea inmind, the prestraining and strain aging in accordance with the presentinvention must be done in the direction in which it is desired that thebody shall have super strength.

The term in the direction as so used in intended to distinguish not onlybetween compression and tension on the one hand, and also between righthand and left hand torsion, but also between end-wise force (i.e.compression or tension) on the one hand, and torsion in either directionon the other. This term is used in this manner throughout thisapplication and in the appended claims.

The prestraining in accordance with the present invention is furtherintended to be restricted to a plastic prestrain, i.e. the applicationof sufficient force so as to effect a strain beyond the elastic limit ofthe material, so that there will be a permanent deformation in the bodydue to and following the prestrain step when the applied force isrelieved. This permanent deformation should be of the order of magnitudeof about 1 to 6% of the original dimension of the part in question inthe direction of the strain and as a permanent strain or deformation tothis extent. It can be applied by any suitable apparatus having thenecessary strength and gripping means to apply the force in question inthe desired direction.

If a body he merely prestrained (without strain aging), for example, intension in accordance with the teachings herein given and be testedimmediately thereafter in tension, the new 1% offset yield strengthapproximates the stress level at which the prestrain was terminated. If,however, a sufiicien-t time is left following the act of prestraining togive what is known as strain aging, then the desirable effects of theprestraining will be present. This time and the temperature at which thestrain aging is accomplished again are not exactly definite. The strainaging apparently takes place much more rapidly as the temperature israised and hence is preferably done at an elevated temperature, eventhough it is theoretically possible to effect strain aging at roomtemperature if sufiicient time is provided. However, due to the desireto secure the results to be attained in a reasonable and limited amountof time, it is ordinarily preferred to use an elevated temperature forstrain aging plus a sufiicient time. This elevated temperature, however,should not exceed the prior tempering temperature used on the same bodywithout undesired results, in effect eliminating all the desirableresults which are sought incident to prestraining and strain agingcombined. The elevated temperature for strain aging is preferably aboutP. less than the tempering temperature. This 50, however, is notnarrowly critical, it being important merely that the strain agingtemperature be somewhat and preferably substantially less than thetempering temperature. It has been found that a 50 difference is apreferred differential in this respect. At temperatures of about 50 lessthan the tempering temperature, strain aging can occur to a satisfactorydegree in about two hours, so that increased time beyond two hours doesnot result in any substantial improvement in the results attained.Again, the two hour period is not narrowly critical, as a greater periodmay be used with impunity, while somewhat lesser periods of time oftenattain a large amount of desirable results sought.

The present invention does not rely upon any particular theory as towhat takes place during strain aging It is believed, however, thatstrain aging is really a diffusion controlled process in which certainsolute atoms such as carbon and nitrogen migrate toward high stressregions created during and as a result of prestrain. It is furtherbelieved that the rate of such migration or ditfusion is approximatelydoubled for every 10 C. increase in temperature at which the strainaging is conducted.

The foregoing theory, which is believed to be correct but is notspecifically relied upon, tends to explain why strain aging operatesbetter at higher temperature values up to about 50 F. below thetempering temperature and also why it can operate even at roomtemperature, which, for the purpose of the present invention, may beassumed to be F. The preferred range for the strain aging temperature isabout 50 F. to about F. below the tempering temperature and mostpreferably about 50 F. below the tempering temperature.

One result of the prestraining and strain aging as aforesaid is thatsteel samples acquire a definite'and relatively high yield point as isevidenced from a comparison of curves A and B of FIG. 1 of the drawingswherein the sample tested to give curve A had been tempered in aconventional manner, but had not been prestrained and strain aged; Whilethat to give curve B had been tempered, then prestrained and strainaged.

It is recognized that prestraining and strain aging has been describedto some extent in the article hereinabove referred to by Stevenson etal. in the Transactions of the American Society of Metals. In the testsset out in this article, not only was the type of steel inappropriatefor maximum desirable results in accordance with the present inventionin that it had a too low nickel content and a too high chromium content;but also the authors of this article did not know or in any case had notinvestigated and told of the necessity that the strain aging temperatureof retempering, as it was called in that article, should besubstantially below the temperature at which the body was firsttempered. For this reason, therefore, they did not succeed in obtaininga strengthening of the steel to the ranges which they desired, i.e.,over about 300,000 p.s.i., accompanied by reasonable ductility. Asaforesaid, they reported a most complete loss of elongation which is ofcourse a measure of ductility. As compared to this, by the selection ofa proper composition, even in the relatively broad range herein set outas appropriate to the present invention, plus the conduct of thestraining and strain aging step at a temperature below that of theoriginal tempering step, the desired characteristics of ductility areretained for the most part, while tremendously improving the strengthsof the bodies being produced. While most of the tests hereinafterreported are in tension, and indicate increased strength in tension dueto prestraining in tension and subsequent strain aging, similar resultswill be obtained in compression or in right-hand torque or in left-handtorque. Thus, for example, if a body or article is to be used towithstand left-hand torque,

it is prestr-ained in left-hand torque, then strain aged so as to give afinal product which is improved as to its ability to stand left-handtwisting or torque. This body will not, however, be significantlyimproved in its resistance to right-hand torque. If, on the other hand,a body Table 2 EFFECT OF PRESTRAINING AND STRAIN A GING ON THE TENSILEPROPERTIES OF STEELS (0.357-INCH DIAMETER TENSILE TEST SPECIMENS) YieldStrength (1,000 p.s.i.) Austenitizing Teinpering Plastic Pre- StrainAging Tensile Elongation Reduction Sample No Temp. F.) Temp. C F.)strain Temp. C F.) Strength (percent in in area (percent) 0.02% offset0.2% otlset (1,000 p.s.i.) 2 in.) (percent) 1 Broke at or outside gagemarks.

The steels reported on in Table 2 above were, after is to be subjectedto right-hand torque during its normal 3.; use, then it is prestrainedin right-hand torque and strain austenitizing, quenched in molten saltat 500 F. (with aged, whereupon its strength to resist or withstandrightthe exception of the sample No. PA-l, which was hand torque ortwisting is greatly improved, but its ability quenched in oil in theconventional manner), held one to withstand left-hand torque or twistingis not significantminute, and then air cooled. The strain aging whencarly improved by the prestraining and strain aging. 40 ried on was fortwo hours at the temperature indicated. In a similar way, prest-rainingand strain aging in ten- The foregoing data is given in full and as thedata was sion enhances the strength of a body to withstand tensiletaken. It is believed, however, that the data as to percent forces;whereas prestraining and strain aging in compreselongation and reductionin area for the sample 2 as sion makes a body stronger to withstandcompressive austenitized at 1550 in erroneous, as this data is substan--forces; but prestraining in one direction does not sigtially out ofline with the remainder of the samples. Annificantly assist instrengthening the body against forces other possible explanation as tothis is that this sample in the or any other direction. This phenomenonis known had a relatively high carbon content and as such is not as theBauschinger effect as aforesaid. within the most preferred range ofcarbon for this gen- The present invention will be better understood bya eral purpose. This may account in part for the relativelyconsideration of a number of actual examples wherein the low ductilityas contrasted with the other samples tested. various embodiments of theinvention will be brought out It is noted, however, that this sample 2had the greatest in detail, tensile strength and the highest 02% offsetyield point. EXAMPLEI These values, however, were attained in thisparticular This example is given to illustrate compositions in sample ata substantlal cost in their ductility. For these cordance with thepresent invention as compared with one F Q themfore, the carbon Q Q 15preferably prior .art composition and to show the results of prestrainWlthm the somwhat Parrower hmlts of flbout and strain aging 0.50%. Thereis attained, however, even 1n sample 2 In Table 1 which follows there isgiven data :as to the when tempered 115mg a relatively highaustenitiling composition of each of five steels which are in accordancep Strength ch31 actel'istics which are beyond those with the presentinvention and a single steel numbered of any other sample tested, sothat this sample is con- PA-1, which is an example of the prior artsteel of the sidered broadly to be within the scope of the presentLadish-type, given for purposes of comparison. invention.

Table 1 COMPOSITION OF THE NICKEL STEELE, WEIGHT PERCENT Sample No. 0 Mns1 1 Ni Cr I oo A1 M0 v i P' l s 0. 50 0. 19 1. s7 3. 003 0. 00s 0. 530. 91 1. 71 3. 77 0. 003 0. 00s 0. 47 0. 1. 77 3. 73 0. 004 0. 009 0.4s 1. 05 1. 92 4. 07 0. 004 0. 00s 0. 49 0. 00 1. 71 3. 68 0. 003 0. 00s0. 47 0. 78 0. 27 0. (s9 0. 005 0. 00s

1 1 EXAMPLE II This example is given to illustrate the present inventionapplied to steels having about 5% nickel. Two actual steels were made upand tested in accordance with former tempering temperature, but suchductility still is substantial and is adequate for most purposes. On theother hand, it is considered undesirable to use a strain agingtemperature or retempering temperature higher than this example, whichare herein numbered as samples 6 5 the onglnal tempenng temperature asthls has resulted m and 7' The composition of these Steels is Set outherein substantial decrease in several of the strength characterbelow inTable 3' istrcs of the product as shown by work reported inthe Table 3prior art publications of Stephenson et al. above referred to.

Percent 10 EXAMPLE III Sample This example demonstrates princ1pally thedesirable O 51 N1 CT V M0 characteristics of a cobalt and/or aluminumcontaining steel and preferably of a steel alloy composition in which F10 :33:: 813g 81%} 16; 3% 8:38 8:}: 8:39 both cobalt and aluminum arepresent. The steels of this character have the characteristic of havinga relatively The balance of these compositions consisted essentiallyhlgh Oflset yleld strength (230,000 or more) of iron with incidentalimpurities, including a small on l p the tests Camed out In amount, lessthan 0.01% each, of sulfur and phos ho termmmg the characteristics of anumber of steels when These samples were tested as before with theresults set 20 tempered at various temperatures in the range of aboutout in Table 4, which follows. 400800 F., a group of steels illustrativeof ditferent type Table 4 RESPONSE OF 5.0% NICKEL sTEELs AND THE EFFECTOF STRAIN AGING TEMPERATURE Yield Strength (1,000 p.s.i.) Sample No.Tampering Plastic Prc- Strain Aging Tensile Elongation Reduction Temp.F.) strain Temp. F.) Strength (percent in in area (percent) 0.02% oilsctYield. Point 0.2% offset (1,000 p.s.i.) 2 in.) (percent) 400 3 350 32533s 33s 33s 5 33 400 3 350 305 342 342 342 e 33 The table above showsthe strength of the as-tempered bodies made of the respectivecompositions and also gives the value for a yield point as such, wherethe yield point is definite and ascertainable. In several instancesduplicate samples were run under the same conditions which gives a goodapproximation of the extent to which the values may be duplicated inrepeated tests. It will be noted that in each instance there is asubstantial retention of ductility as measured by the elongation andreduction in area, even though the very substantial strengtheningincident to the practice of the process of the present inventionresulted in some loss of ductility. The remaining ductility is adequatefor many and most purposes for which this steel is desired and is muchgreater than was present in prior art steels treated in a somewhatsimilar manner. As to each sample, a set of data is also given where thestrain aging temperature is substantially equal to the temperingtemperature. As will be noted, both the elongation and the reduction inarea and hence the ductility under these conditions are less than in theinstances where .the strain aging temperature is F. less than thecompositions were used. The compositions of the several samples testedare given in Table 5, which follows.

Table 5 Of the foregoing, sample 8 is not the preferred form inaccordance with this phase of the invention as it contains substantialamounts of molybdenum and vanadium; even though this steel is broadlywithin the general scope of this invention in that it is quite similarto samples 5 and 6 which have quite desirable properties as to productswhich have been strained and strain aged. Of the four sampleshereinabove given, that designated 10 is approximately the preferredcomposition. The foregoing samples were tested on an as-tempered basiswith the results set out in Table 6, which follows.

Table 6 ROOM TEMPERATURE TENSILE PROPERTIES OF EXPERIMENTAL STEELS l(0.357-INCH ROUND SPECIMENS) Yield Strength Elongation Sampl Tampering(L000 Tensile Reduction 0. Temp. C F.) Strength in area v 0.02% 0.2%(1,000 p.s.i.) Percent in Percent in (percent) offset offset 2 in. 1 in.

1 Austenitized at 1,500 F., quenched into molten salt at 500 1K, held 1minute and air cooled.

From the foregoing it will be seen that the compositions of the presentinvention provide quite high 0.2% offset yield points which are,however, substantially below the ultimate tensile strengths in mostinstances, but tend to approach them, particularly as to the compositionof sample 10. It is also clear that increases in tempering temperaturegenerally result in a reduction in tensile strength. However, when thesedata are plotted, it will be noted that the unusual results shown inFIGS. 2 and 3 of the drawings ensue, particularly in that, referring toFIG. 2, there is a peak of 600 F. for 0.2% offset yield point, which ishigher than the corresponding yield point for the same samples temperedat 400 F. 'In the drawings, curves C and C of FIGS. 2 and 3 are plots ofthe data given as to sample 10; curve D of FIG. 2 and D of FIG. 3 is aplot of the data given for sample 9, which is next preferred; curve E ofFIG. 2 and E of FIG. 3 is a plot of the data given for sample 11; whilecurve F of FIG. 2 and F of FIG. 3 is a plot of the data for sample 8.The dotted line curves of FIGS. 2 and 3 are plots of the correspondingdata for a prior art composition which is generally known as Type 4150steel, the composition of which is given hereinabove.

It will also be noted from Table 6 that as to the preferred composition,i.e. that of sample 10, there is a second relatively low peak (at240,000 p.s.i.) for the 0.2 offset yield strength versus temperingtemperature at a tempering temperature of 800 F. in addition to the peakat 600 F. shown on FIG. 2.

Another and' very impressive demonstration of the particulardesirability of the composition of sample 10 is the fact that :a 12 inchlong button-headed tensile bar of this composition having a diameter of0.419 inch (less than inch) when used in a test demonstration supportedthe weight of a 45,000 lbs. freight car while serving as its solesupport by being interposed between a rigging secured to the car on theone hand and a hook from a heavy crane above the car on the other hand.In this demonstration, the test piece was subjected to a stress of325,000 p.s.i. In an earlier test on this same piece, to ascertainwhether it could stand not only the weight of the car, but also theextra stress incident to lifting the freight car several feet off therails, it was subjected to a tensile test wherein it sustained a totalpull of 50,000 lbs., which was equivalent to 360,000 p.s.i. This testpiece had Percent Carbon 0.53 Manganese 0.90

Silicon 1.60 Nickel 3.60 Chromium 0.30 Cobalt 2.00 Aluminum 0.75Phosphorus 0.006 Sulfur 0.006

and the balance iron with incidental impurities.

EXAMPLE IV This example is to illustrate the fracture toughnesscharacteristics of steels having compositions generally in accordancewith the present invention and further to set out preferred compositionswithin the general scope of the compositions of the present inventionwhich have the best characteristics of fracture toughness.

In order to determine fracture toughness, it is usual at this time todetermine the tensile properties of sharply notched steel sheet. In someinstances the steel sheets used in tests of this kind are notched on thesides, with the notches facing one another and are imperforate in thecenter portion to be tested, i.e. between the ends that are gripped inthe testing apparatus. It is quite common, however, that the endportions of a sample to be gripped in the testing apparatus may beperforated for a large bar so as to facilitate the gripping of such endportions. A discussion of the fracture testing of notched specimens iscontained in the ASTM Bulletins for January and February 1960, pages 29et seq. in the January issue and page 18 et seq. in the February issue.1

In the test work done in connection with the present invention, however,a sample as particularly shown in FIG. 4 of the drawings was used, thisfigure giving exact dimensions of the test piece which included endportions having perforations of 0.500 inch for gripping purposes 5 and acentral hole with the notches disposed on diametrically oppositeportions thereof and extending outwardly from the center hole. Thecentral hole and the notches therein were formed by an electrojetapparatus, i.e. a tool employing an electric arc and operating under anoil coolant. The. specimens were then fatigue-cracked at the notchesprior to testing, the notches serving as fatigue crack starters. Thiswas done by repeatedly alternately applying a high tensile stress equalto about one fifth of the yield strength of the material and a lowtensile stress equal to about one-half the high tensile stress forstarting and propagating fatigue cracks. These stresses were appliedintermittently for about 30,000 cycles. This was carried on until thefatigue cracks as shown at 11 and 12, FIG. 4, had a total overall lengthas shown by the dimensions on that figure of about 0.750 inch from endto end. In order thereafter to determine the length attained by theslowly propagating crack at the inception of crack instability, Indiaink was applied to the notches near the apex of each notch so as tofollow up the crack. This length may in turn be used to compute a valueknown in the art as fracture toughness. The ink, while wet and fiowable,followed end crack as it was gradually extended up to the point wherecrack instability set in. This technique was used for estimating thelength of the slowly propagating crack and from this, fracture toughnessvalues can be computed. This type of testing is that favored by the ASTMCommittee on the Fracturing of High Strength Steel Sheet at its meetingof November 17, 1960.

The composition (in percent by weight) of various steels which have beentested or known data as to which is herein given for comparison purposesare as follows: (the PA steels being compositions known in the priorart) Table 7 Sample No. 0 Mn Si Ni Cr M0 V 00 Samples were prepared fromthe foregoing materials set out in Table 7, the samples being cut ineach instance first with the long axes of the samples, i.e. the verticaldirection of the drawing as seen in FIG. 4, extending longitudinally ofthe roll sheet (i.e. longitudinally of the direction of rolling). Ineach instance the sample was cut out from the sheet and the holes andnotches formed therein prior to conventional austenitizing, quenchingand tempering, all of which was then carried on with the formed andpunched sheet and then the fatigue cracks formed therein by thealternate application of high and low loads in a longitudinal directionof the sample, the high load being about twice the low load. It is foundthat the amount of loading and the number of cycles required to formsuch fatigue cracks are a function of the frequency of the alternationbetween high and low loading and hence the data as to the manner as towhich these test pieces was fatigue-cracked is not per secharacteristic. Sufiice it to say that the fatigue cracks plus thecenter hole diameter and the depths of the notches extended initially adistance of about 0.75 inch as set out in the drawing, FIG. 4. Thesamples were then tested at room temperature in tension and the maximumtension determined up to the point of crack instability. 75

The length of the crack at the point of crack instability was alsodetermined by the ink technique as above described, as the ink followsthe crack propagation up to the point of crack instability. The netnotch strength for each sample determined from the above test wascalculated as the maximum applied load divided by the uncracked crosssection area of the sample at the onset of unstable crack propagation.

As so tested, sample 13, which is substantially the preferredcomposition in accordance with the present invention for resistanceagainst crack propagation or fracture toughness as it is sometimescalled, showed a net notch strength averaging 246,000 p.s.i. for anaverage of three specimens tested, the actual values for these specimensbeing 230,000, 248,000 and 261,000 and a fourth specimen also testedgiving the value of 258,000. These specimens were tempered at 400 andhad varying thicknesses between 0.096 inch for the first three inquestion and 0.116 inch for the sample having the 258,000 p.s.i. testresults. In each instance the initial crack length was between 0.750inch and 0.780 inch and the final crack length also varied from 1.17inch to 1.27 inch. These figures for notched tensile strength may becompared with the 0.2 offset yield strength of smooth bar test pieces ofthe same composition of about 210,000-220,000 p.s.i. and a total tensilestrength of about 260,000 p.s.i., also for smooth round bar test piece.

These results may further be compared with that of the prior artcomposition hereinabove given as sample PA3 wherein a similar test pieceshowed a notched tensile strength of only 189,000 p.s.i. Again, samplePA-S, which is a steel of the 4340 type, averaged about 206,000 p.s.i.for net notch strength (calculated as aforesaid), with different samplestested giving various values between 202,- 000 and 209,000 p.s.i. Thenet notch strength values of the steel according to the presentinvention may also be compared with the figures ascertaina'ble fromprior art literature as to the composition given as PA-6, which was thebest of the prior art samples and had a net notch tensile strength of218,000 p.s.i.

The characteristics of the failure of the several samples is also ofgreat interest. Those having compositions according to the presentinvention, when cut as longitudinally extending samples, were 100% shearfailures, which is the desired condition. As contrasted with this, thesamples of the several prior art compositions were, in practically allinstances, substantially less than 100% in shear, values of 73%, andbeing common for different samples.

Some samples of the preferred composition wer also treated in thefollowing manner, sometimes known as hot-cold working. This involvedaustenitizing the material in sheet form at 1450 F., then. quenching inmolten salt at 600 F. and immediately thereafter and prior to thecooling of the sample, rolling in a single roll pass to as to reduce thethickness of the sheet by 35%, followed by quenching in oil at about F.The samples as thus prepared were then tested to give relatively low netnotch tensile strength results of about 87,000 p.s.i. When, however,samples prepared in this way up to the point of testing were furthertempered at 400 F; (all these samples in accordance with the presentinvention having a thickness between 0.054 and 0.060 inch), the testresults showed net notch tensile strength values of 239,000 and 262,00p.s.i. with final cracked lengths of 1.30 inch and 1.32 inchrespectively as against final crack lengths for the first group ofsamples having net notch strengths of 87,000 p.s.i. of only 0.82 and0.87 inch respectively.

The importance of steel capable of withstanding notchtype tests asaforesaid is believed to be very great in that such cracks have a verysmall root radius, of 0.001 inch or less. Cracks of this nature could beformed in the manufacture of many articles from sheet steel where it isdesired that the article shall have great strength and shall not failunder extreme operating conditions. Such cracks 17 could be generatedduring heat treating or welding or other fabrication operations in thecourse of manufacturing articles from sheet steel. Cracks of this kindmay not be readily detectable, even if detectable at all, during normalinspection. For these reasons, therefore, resistance to crackpropagation is often considered of greater im portance than mere tensilestrength values of the steel.

Samples 14 and 15 above referred to were tested at both 400 and 600 F.tempering temperatures. The tests made at 400 showed that a sample ofthe type shown in FIG. 4 and 0.087 inch in thickness had a 100% sheartype failure and had a net notch strength as hereinabove defined of250,000 p.s.i. Sample 15, also tempered at 400 F., was similarly testedwith the sample formed as shown in FIG. 4 and with a thickness of 0.086inch. The failure in this instance was only 58% of a shear type and thenet notch strength only 155,000 p.s.i. This indicates that cobalt at the2% level is completely tolerable. However, the carbon-cobalt balance insample No. 15 is somewhat high and consequently samples of thisthickness range did not break with full shear failures and high netnotch strengths.

When samples 14 and 15,-each of a size and configuration as shown inFIG. 4 and 0.086-0.087 inch in thickness, were tempered at 600 F., thesamples broke with full shear failures and high net notch strengths(about 250,000256,000 p.s.i.). This indicates that at the highercobalt-carbon balance and at the higher tempering temperatures, fullshear failures and high net notch strengths may be achieved, even at theaforementioned thickness. The compositions of samples 14 and 15, whenformed into smooth, round tensile bars 0.357 inch in diameter andtempered at 600 F., when tested at room temperature, had

0.2 otfset yield strengths of about 235,000-237,000 p.s.i.

and tensile strengths of about 255,000-257,000 p.s.i. It is thus shownthat all the net notch strengths for the 600 F. tempered samples Nos. 14and 15 were substantially in excess of the smooth bar yield strengths.

Another steel alloy composition which has been shown to have good notchproperties, in that it is highly resistant to crack propagation of anotched sample as aforesaid, is one having 0.48% carbon, 0.45%manganese, 0.25% silicon, 5.10% nickel, 0.30% chromium, 0.30%molybdenum, 0.12% vanadium and 0.006% (max.) each of sulfur andphosphorus with the balance iron with incidental impurities.

While there has been disclosed herein a relatively broad range of mediumcarbon alloy steels in accordance with the present invention, certainmore limited ranges for particular purposes, and specific compositionsalso for particular purposes, all as set out herein, and while a processof treating steel to attain super strengths in a particular directionhas also been disclosed, other variations and equivalents of theforegoing will become evident to those skilled in the art based uponthis disclosure and the appended claims. I do not intend to be limited,therefore, except by the scope of the appended claims, which are to beconstrued validly as "broadly as the state of the art permits.

What is claimed is:

1. The method of anisotropically strengthening an alloy steel article toimpart maximum strength in a preselected direction of said article withretention of good ductility, said article being made of a steelconsisting essentially of about 0.35-0.6% carbon, 37% nickel, 0.2-0.5%chromium, up to 2% each of manganese and silicon, up to 0.5% molybdenum,up to 0.2% vanadium, up to 5% cobalt, up to 1% aluminum, and the balancesubstantially all iron, said method comprising the steps ofaustenitizing said steel article at temperature above 1400 F. andthereupon quenching and tempering at strain-aging said article byheating and holding at elevated temperature not substantially higherthan the temperature at which said article was tempered and for a periodrequired to impart a preselected combination of strength and ductility.

2. The method according to claim 1 wherein the prestraining of saidarticle in said preselected direction is produced by tensioning saidarticle in said direction thereby to produce a permanent elongation ofsaid article in said direction prior to strain-aging thereby to impartmaximum tensile strength in said direction upon strain-aging.

3. The method according to claim 1 wherein the prestraining of saidarticle in said preselected direction is produced by compressing saidarticle in said direction thereby to impart a permanent compression ofsaid article in said direction prior to strain-aging and thereby toimpart maximum compression strength in said direction upon strain-aging.

4. The method according to claim 1 wherein said prestraining is such asto produce a permanent deformation of said article in said preselecteddirection of about 23% of its original length in said direction.

5. The method according to claim 1 wherein said tempering is conductedat about 400 F. and wherein said strain-aging is conducted at aboutBOO-350 F.

6. The method according to claim 1 wherein said tempering is conductedat about 400 F., wherein said prestraining is such as to impart apermanent deformation of said article of about 3% of its original lengthin said preselected direction, and wherein said strain-aging isconducted at about 350 F. to produce an 0.2% offset yield strength aboutequal to the tensile strength in said preselected direction of saidarticle.

7. The method of anisotropically strengthening an alloy steel article toimpart maximum strength in a preselected direction of said article withretention of good ductility, said article being made of a steelconsisting essentially of about 0.350. 6% carbon, 3-7% nickel, 0.2-0.5%chromium, up to 2% each of manganese and silicon, up to 0.5% molybdenum,up to 0.2% vanadium, 2 to 5% cobalt, up to 1% aluminum, and the balancesubstantially all iron, said method comprising the steps ofaustenitizing said steel article at temperature above 1400" F. andthereupon quenching and tempering at 8. The method according to claim 7wherein the-prestraining of said article in said preselected directionis produced by tensioning said article in said direction thereby toproduce a permanent elongation of said article in said direction priorto strain-aging thereby to impart maximum tensile strength in saiddirection upon strain-aging.

9. The method according to claim 7 wherein the prestraining of saidarticle in said preselected direction is produced by compressing saidarticle in said direction thereby to impart a permanent compression ofsaid article in said direction prior to strain-aging and thereby toimpart maximum compression strength in said direction upon strain-aging.

10. The method according to claim 7 wherein said compression is such asto produce a permanent compression of said article in said preselecteddirection of about 2- 3% of its original length in said direction.

11. The method according to claim 7 wherein said tempering is conductedat about 400 F. and wherein said strain-aging is conducted at about300350 F.

12. The method according to claim 7 wherein said tempering is conductedat about 400 F. wherein said prestraining is such as to impart apermanent deformation of said article of about 3% of its original lengthin said preselected direction and wherein said strain-aging is conductedat about 350 F. to produce an 0.2% offset yield strength about equal tothe tensile strength in said preselected direction of said article.

13. An article made of a quenched and tempered alloy steel consistingessentially of about: 0.35-0.6% carbon, 37% nickel, 02-05% chromium, upto 2% each of manganese and silicon, up to 0.5 molybdenum, up to 0.2%vanadium, up to 5% cobalt, up to 1% aluminum, and the balancesubstantially all iron, characterized in having an ultimate strength ofat least 225,000 p.s.i., an 0.2% offset yield strength of at least200,000 psi. and a tensile elongation of at least 10% in one inch.

14. An article made of a quenched and tempered alloy steel consistingessentially of about: 0.350.6% carbon, 37% nickel, 0.20.5% chromium, 25%cobalt, up to 2% each of manganese and silicon, up to 0.5% molybdenum,up to 0.2% vanadium, up to 1% aluminum, and the balance substantiallyall iron, characterized by an 20' ultimate strength of at least 225,000p.s.i., an 0.2% offset yield strength of at least 200,000 psi, and atensile elongation of at least 10% in one inch.

15. An article made of a heat-treated alloy steel consisting essentiallyof about: 0.35-0.6% carbon, 37% nickel, 0.2-0.5 chromium, up to 5%cobalt, up to 2% each of manganese and silicon, up to 0.5% molybdenum,up to 0.2% vanadium, up to 1% aluminum, and the balance substantiallyall iron, said article having maximum anisotropic strength properties inone direction thereof and having in said direction ultimate and 0.2%ofifset strengths of at least 300,000 p.s.i., and a tensile elongationof at least 4% in two inches.

References Cited by the Examiner UNITED STATES PATENTS DAVID L. RECK,Primary Examiner.

1. THE METHOD OF ANISOTROPICALLY STRENGTHENING AN ALLOY STEEL ARTICLE TOIMPART MAXIMUM STRENGTH IN A PRESELECTED DIRECTION OF SAID ARTICLE WITHRETENTION OF GOOD DUCTILITY, SAID ARTICLE BEING MADE OF A STEELCONSISTING ESSENTIALLY OF ABOUT 0.35-0.6% CARBON, 3-7% NICKEL, 0.2-0.5%CHROMIUM, UP TO 2% EACH OF MANAGANESE AND SILICON, UP TO 0.5%MOLYBDENUM, UP TO 0.2% VANADIUM, UP TO 5% COBALT, UP TO 1% ALUMINUM, ANDTHE BALANCE SUBSTANTIALLY ALL IRON, SAID METHOD COMPRISING THE STEPS OFAUSTENITIZING SAID STEEL ARTICLE AT TEMPERATURE ABOVE 1400*F. ANDTHEREUPON QUENCHING AND TEMPERING AT ABOUT 350-600*F., THEREUPONPRESTRAINING SAID ARTICLE IN SAID PRESELECTED DIRECTION TO IMPARTTHERETO A PERMANENT DEFORMATION IN SAID DIRECTION OF ABOUT 1-6% OF THEORIGINAL LENGTH OF SAID ARTICLE IN SAID DIRECTION, AND THEREAFTERSTRAIN-AGING SAID ARTICLE BY HEATING AND HOLDING AT ELEVATED TEMPERATURENOT SUBSTANTIALLY HIGHER THAN THE TEMPERATURE AT WHICH SAID ARTICLE WASTEMPERED AND FOR A PERIOD REQUIRED TO IMPART A PRESELECTED COMBINATIONOF STRENGHT AND DUCTILITY.